[Mesa-dev] [PATCH] radv: emit fmuladd instead of fma to llvm.

[Ilia Mirkin]

Wouldn't this guarantee that nothing is fused (and thus fine)?
Presumably fmuladd always does mul+add either as 1 or 2 instructions?

On Wed, Oct 4, 2017 at 10:57 AM, Connor Abbott <cwabbott0 at gmail.com> wrote:
> If the fma has the exact flag, then we need to use the llvm.fma
> intrinsic. These come from fma() calls with the precise or invariant
> qualifiers in GLSL, where you basically have to fuse everything or
> fuse nothing consistently, and llvm.fmuladd doesn't guarantee that.
>
> On Tue, Oct 3, 2017 at 10:10 PM, Dave Airlie <airlied at gmail.com> wrote:
>> From: Dave Airlie <airlied at redhat.com>
>>
>> For Vulkan SPIR-V the spec states
>> fma() Inherited from OpFMul followed by OpFAdd.
>>
>> Matt says the backend will do the right thing depending on the
>> hardware being compiled for, if you use the fmuladd intrinsic.
>>
>> Using the Mad Max pts test, on high settings at 4K:
>> CHP: 55->60
>> HGDD: 46->50
>> LM: 55->60
>> No change on Stronghold.
>>
>> Thanks to Feral for spending the time to track this down.
>>
>> Signed-off-by: Dave Airlie <airlied at redhat.com>
>> ---
>>  src/amd/common/ac_nir_to_llvm.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/src/amd/common/ac_nir_to_llvm.c b/src/amd/common/ac_nir_to_llvm.c
>> index d7b6259..11ba487 100644
>> --- a/src/amd/common/ac_nir_to_llvm.c
>> +++ b/src/amd/common/ac_nir_to_llvm.c
>> @@ -1707,7 +1707,7 @@ static void visit_alu(struct ac_nir_context *ctx, const nir_alu_instr *instr)
>>                                                       result);
>>                 break;
>>         case nir_op_ffma:
>> -               result = emit_intrin_3f_param(&ctx->ac, "llvm.fma",
>> +               result = emit_intrin_3f_param(&ctx->ac, "llvm.fmuladd",
>>                                               ac_to_float_type(&ctx->ac, def_type), src[0], src[1], src[2]);
>>                 break;
>>         case nir_op_ibitfield_extract:
>> --
>> 2.9.4
>>
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